In optical communications systems data is imparted onto light by varying the intensity and/or phase of the light signal. In a simple example binary data may be signaled by emitting light of maximum intensity to represent a “1” and zero intensity to represent a “0”. An optical format of this kind, where the data is represented by the amplitude of the signal, is known as amplitude shift keyed (ASK).
In addition to ASK formats, it is known to transmit over long haul optical communications using phase-shift keyed (PSK) formats. Examples of PSK formats include differential PSK (DPSK) and differential quadrature PSK (DQPSK). PSK formats impart information onto the optical signal by varying its phase (rather than its intensity).
Data transmitted optically is typically manipulated in electronic form at its destination. Optical receivers for use in optical communications systems are therefore capable of converting optical signals into electronic signals. This is a relatively simple process for ASK formats, where components such as photo-diodes) for example, positive intrinsic negative (PIN) diodes) can be used to generate a signal which is proportional to the intensity of the received light. However, such components are typically incapable of discriminating between the phase of incident signals, and so additional features are required to decode PSK formats.
In particular, optical discriminators (differential delay interferometers) are used to extract the information included in PSK signals. Discriminators of this type typically split an incoming PSK signal into two components, and apply a relative delay to one of these components before they are recombined. By setting the delay as an integer number of the time period for a data bit in the signal, an initial bit can be compared with a subsequent bit through the interference of the two when the components are combined. The overall amplitude of the signal will consequently represent the difference in phase between these two bits. Since the amplitude can be measured by conventional components, the difference between the two bits can then be inferred from the resultant signal.
Reception of DPSK data requires accurate relative wavelength tuning control between the transmit signal carrier wavelength and the receiver optical discriminator such that the two remain accurately locked to each other. It is not possible to fix the tuning point(s) since the practical inherent stability of the transmitter laser source and the receiver discriminator cannot be relied upon to maintain good performance.
It is known, and conventionally preferred, to keep the transmit laser wavelength nominally fixed and to lock the receiver discriminator by a feedback control loop that measures receiver performance. An example of such a device is shown in US patent application US 2006-133827, which is incorporated herein by reference. The device in this application uses a dither based control loop which adjust the temperature experienced by one of the separated components passing through the discriminator, thereby altering the relative delay experienced by the two components before they are combined. The electrically detected radio frequency (RF) peak voltage as measured at a receiver photodiode is measured for higher and lower temperatures, and the temperature of the discriminator is adjusted towards the temperature in which a higher peak voltage is measured. Eventually, the device reaches an equilibrium position where alteration of the temperature would be disadvantageous in either direction, and the discriminator may be considered tuned to the carrier frequency of the optical signal.
Although thermal control of the discriminator wavelength described above may lead to improved results, it has an inherent and usually long time constant. Furthermore, as the direction in which to tune the discriminator is also established by variation of the discriminator control temperature, the thermal time constant of the discriminator also limits the speed at which this can be done. The combination of a thermal time constant and the requirement of dithering, means control is cumbersome and slow.
Moreover, further difficulties occur in practice as discriminators usually incorporate a heater rather than a thermo-electric cooler (TEC), for component fabrication simplicity, and as a result the dithering process relies on temperature decay to set the upper dither rate (that is, the process is even slower than would be the case for a device that could be actively cooled).
It is also found that to achieve a dither amplitude that offers a satisfactory results, a significant varying heat flux is required. This can lead to premature component failure. This is compounded by the fact the heater has to run at an elevated temperature with respect to the ambient in order to produce a tuning reaction—the higher the temperature with respect to ambient the faster the temperature decay for dither, but also the more component stress. As well as limiting the effectiveness of these techniques in adjusting the discriminator during use, the slow control loop can also hinder start-up time from cold before satisfactory transmission can be achieved.